Field of the Invention
The quality and mass flow rate of wet steam through a pipeline or conduit are characteristics which are not readily determined with a high degree of accuracy. Many commercial instruments and forms of apparatus are capable of furnishing an analysis of steam quality and mass flow rate. This equipment, however, normally embodies a deterrent toward accuracy of measurement.
It has been determined as a commercial reality that the accurate measurement of steam characteristics such as flow and quality can be accomplished through the facility of a variety of different instruments or combination of instruments. However, when the steam measurement provides a satisfactory answer, but will constitute an adverse affect to the ultimate use of the steam, the procedure is usually not acceptable.
For example, steam rate of flow over into a large scale production such as injection into a subterranean crude oil holding reservoir, production can be successfully accomplished through the use of instrumentation involving a critical flow choke and an orifice plate. This is illustrated in U.S. Pat. No. 4,836,032 (Redus, et al.) wherein the disclosed steam measuring system provides means for initially passing the steam flow through an orifice, and thereafter, through an elongated critical flow choke. The resulting measurable characteristics of the steam will allow an accurate calculation of both the steam flow rate and its quality. This system, however, falls short of practicality in regard to the ultimate steam pressure, a factor prompted by the characteristics of the patentees' critical choke which automatically reduces the downstream pressure of the steam by about 50% from the upstream pressure.
In a similar steam measuring application of the prior art, U.S. Pat. No. 4,576,043 (Nguyek), the patentee utilizes the combination of an orifice plate positioned upstream of a venturi to determine one or more flow rate parameters of a two phase flow. This two phase flow introduces a condition that does not equate it with other comparable steam flow conditions as where a critical steam flow is required by the measuring instrument. More particularly, two phase flow cannot be equated to the type of flow presently under consideration is steam at critical flow conditions.
One such steam monitoring and measuring instrument is disclosed in U.S. Pat. No. 4,836,032, dated Jun. 6, 1988. In the disclosed apparatus, the patentees provide a method and apparatus for measurement of wet steam quality and mass flow rate. This is achieved by passing the wet steam through a conduit in which an orifice plate is disposed axially upstream of a constant bore diameter critical flow choke.
In summary, a primary fault inherent to an arrangement disclosed by Redus, et al., resides in the relatively large decrease in steam pressure realized at the downstream side of the measuring instrument. This pressure loss, when considered for high volume usage, can achieve an order of magnitude of 50% of the pressure at the measuring apparatus inlet. The loss is attributable at least in part to the nature of the patentees use of a critical choke which is characterized by a constant diameter flow passage.
An exaggerated pressure loss of this magnitude can be readily tolerated in many instances and applications. However, where the steam flow is delivered to a point of downstream use after being so monitored, a severe pressure drop across the measuring instrument could constitute a detriment. Further, the lower resulting pressure might preclude use of an instrument or equipment that could otherwise be utilized.
BSI toward obtaining an instrument or apparatus for more accurately monitoring and measuring a pressurized steam flow through a conduit or the like without realizing a substantial pressure loss, the present combination is provide. The novel apparatus includes primarily a conductor, preferably round in cross-section, such as a conduit, pipe, tubing or the like which defines a main steam flow passage.
The main stream flow passage is communicated with a pressurized source of the steam. The latter normally includes necessary valving that allows a regulated flow of the steam within a desired pressure range, to be delivered downstream of the measuring instrument.
The steam conduit is provided with a transversely positioned plate having a constricted, orifice opening therethrough. The orifice plate is located upstream of, and spaced from a critical flow venturi defining a chamber therebetween.
A multi-segment, or composite flow passage extending through the critical flow venturi includes an inlet or upstream segment defined by a progressively narrowing cross-section or progressively converging wall segment which terminates at a constricted throat. From the throat, the steam gradually expands along a smooth surfaced, frusto conical wall formed by a constantly diverging cross-section which defines the downstream segment of the venturi composite flow passage.
To monitor varying factors along the steam path, pressure gauges or sensors are communicated with the steam flow passage. A fist gauge or pressure sensing means is positioned across the orifice plate to register pressure differential. A second pressure gauge or sensing means is positioned in communication with the intermediate chamber to constantly sense pressure conditions therein. A third pressure sensor communicates with the constricted throat immediately downstream of the smallest diameter of the latter.
The configuration of the venturi flow passage allows steam flowing along the walls thereof to substantially maintain its pressure prior to being discharged from the passage's discharge or exit port. With the inlet steam pressure known, independent equations for mass flow rate and steam quality can thereafter be solved simultaneously utilizing these pressure factors or values derived from the steam flow measurement.
It is, therefore, an object of this invention to provide a monitoring apparatus capable of providing accurate steam quality and mass flow rate measurements of a pressurized steam flow while maintaining a minimal pressure drop across the measuring apparatus.